Pumps used in typical high pressure liquid chromatography (HPLC) generate approximately 5000 psi. These pumps, which typically use mechanical mechanisms to actuate flow-generating pistons, cannot create the pressures required for ultra-high pressure liquid chromatography (UHPLC), which operates at pressures in excess of 25,000 psi. Pneumatic amplifiers have been used to generate high pressures for UHPLC systems. For example, in the paper entitled “Ultrahigh-Pressure Reversed-Phase Liquid Chromatography in Packed Capillary Columns,” Analytical Chemistry, 1997,69(6), 983-989, two Haskel pneumatic amplifier pumps are used to perform isocratic LC separations at pressures of 60,000 psi. FIG. 1 shows such a configuration. However, since the gas in pneumatic amplifiers is compressible, pneumatic amplifiers can deliver only constant pressure flow, which precludes high pressure gradient mixing necessary to perform gradient chromatography. That is, in gradient chromatography, two solvents are mixed in varying ratios, thereby requiring constant volume flow of the respective solvents, which cannot be done with pneumatic amplifier pumps. Alternatively, gradient chromatography can be performed by feeding the gradient mixture into a single pump. However, this is difficult to implement at the low flow rates used in UHPLC.
The paper entitled “Ultrahigh-Pressure Reversed-Phase Capillary Liquid Chromatography: Isocratic and Gradient Elution Using Columns Packed with 1.0-m Particles,” Analytical Chemistry, 1999,71(3), 700-708, describes a UHPLC system that uses a mechanical actuator rather than a pneumatic actuator. The mechanical actuator comprises an ultrahigh-pressure constant-flow syringe pump including an electric motor connected to a gear reduction system and linear actuator that moves a piston. Although the syringe pump can generate operating pressures in excess of 130,000 psi, the mechanical actuator is too large to commercialize effectively.